tpm: Handle negative priv->response_len in tpm_common_read()

The priv->response_length can hold the size of an response or an negative
error code, and the tpm_common_read() needs to handle both cases correctly.
Changed the type of response_length to signed and accounted for negative
value in tpm_common_read().

Cc: stable@vger.kernel.org
Fixes: d23d1248 ("tpm: fix invalid locking in NONBLOCKING mode")
Reported-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Tadeusz Struk <tadeusz.struk@intel.com>
Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>

drivers/char/tpm/tpm-dev-common.c

@@ -130,7 +130,7 @@ ssize_t tpm_common_read(struct file *file, char __user *buf,
 		priv->response_read = true;
 
 		ret_size = min_t(ssize_t, size, priv->response_length);
-		if (!ret_size) {
+		if (ret_size <= 0) {
 			priv->response_length = 0;
 			goto out;
 		}

drivers/char/tpm/tpm-dev.h

@@ -14,7 +14,7 @@ struct file_priv {
 	struct work_struct timeout_work;
 	struct work_struct async_work;
 	wait_queue_head_t async_wait;
-	size_t response_length;
+	ssize_t response_length;
 	bool response_read;
 	bool command_enqueued;